This invention relates generally to the casting of metals. More specifically, it relates to shell molds used in the casting of metal components, e.g., components made from superalloys.
The casting of metals is carried out by various techniques, such as investment casting. Ceramic shell molds are used during investment-casting, to contain and shape the metal in its molten state. The strength and integrity of the mold are very important factors in ensuring that the metal part has the proper dimensions. These shell mold characteristics are especially critical for manufacturing high performance components, such as superalloy parts used in the aerospace industry.
Investment casting techniques often require very high temperatures, e.g., in the range of about 1450xc2x0 C. to 1750xc2x0 C. Many conventional shell molds do not exhibit sufficient strength at those temperatures. The molds become susceptible to bulging and cracking when they are filled with the molten metal. (Bulging can also occur when very large parts are being castxe2x80x94even at lower temperatures). Bulging can alter the dimensions of the mold, thereby causing undesirable variation in the component being cast. Cracking could result in failure of the mold as the molten material runs out of it.
Clearly, greater strength and dimensional stability are required for shell molds used at very high casting temperatures, or for those used to cast very large parts. The problem is addressed by J. Lane et al in U.S. Pat. No. 4,998,581. In that disclosure, shell molds are strengthened by wrapping a fibrous reinforcing material around the shell mold as it is being made. In preferred embodiments, the reinforcing material is said to be an alumina-based or mullite-based ceramic composition having a specific, minimum tensile strength. The reinforcing material is apparently wrapped in spiral fashion around the shell mold with a tension sufficient to keep it in place as ceramic layers are applied to the mold to build it up to its desired thickness.
The Lane patent appears to provide answers to some of the problems described above. However, there appear to be some considerable disadvantages in practicing some embodiments of the invention disclosed in that patent. For example, mullite-based materials are difficult to produce without second phase inclusions of either silica- or alumina-containing compounds. These inclusions can degrade the physical properties of the mold. In addition, many of the reinforcing materials employed in U.S. Pat. No. 4,998,581 have thermal expansions much less than the thermal expansion of the mold. These large thermal expansion differences will make fabrication of a crack-free mold more difficult.
It should thus be apparent that further improvements in the properties of shell molds used under the conditions described above would be welcome in the art. The shell molds should have the strength to withstand high metal-casting temperatures, and should be suitable for casting large parts. The molds should also be dimensionally stable at elevated temperatures, and throughout various heating/cooling cycles. Moreover, if the molds are to be improved by the use of reinforcing materials, such materials should be flexible enough, before being fired, to satisfy the shape requirements for the mold, especially when intricate metal components are being cast. Finally, the preparation of improved shell molds should be economically feasible, e.g., not requiring the use of a significant amount of additional equipment. The use of the new molds should not result in undesirable increases in the cost for manufacturing metal parts in the investment casting process.
The present invention satisfies many of the needs described above. In one aspect, the invention embodies a ceramic casting shell mold which has a pre-selected shape. The shell mold comprises repeating layers of a ceramic material which define the wall thickness and shape of the mold. A key feature of this embodiment is that at least one of the layers of ceramic material contains whiskers which provide structural reinforcement to the shell mold. The whiskers are formed of a refractory material, such as an alumina-based material.
In preferred embodiments, the whiskers are incorporated into a layer of the ceramic material which is disposed at a position off-center of the wall-thickness of the mold, e.g., a slurry layer which is situated within about 10% to about 40% of the thickness from the inner wall of the mold. Frequently, at least two of the layers of ceramic material contain whiskers. In preferred embodiments, the whiskers in one of the adjacent layers are out of alignment with the whiskers in the other adjacent layer, e.g., are oriented at an angle of about 60 to about 90 degrees relative to the whiskers in the other adjacent layer.
Another embodiment of this invention is directed to a method for making a ceramic casting shell mold, comprising the steps of:
(I) applying a slurry which comprises ceramic-based whiskers to a ceramic layer-surface of a partial shell mold formed by applying successive ceramic layers over one another, forming a whisker-containing ceramic layer;
(II) completing the shell mold so that it has a desired wall-thickness, by applying additional ceramic layers over the whisker-containing ceramic layer; and then
(III) firing the shell mold at an elevated temperature.
As described below, the whisker-containing ceramic layer is usually disposed at a position off-center of the wall-thickness of the mold. Moreover, more than one of the ceramic layers can contain whiskers. When whiskers are in multiple ceramic layers which are adjacent to one another, or relatively close to one another, the whiskers are preferably oriented so that they are out of alignment with the whiskers in adjacent or nearby layers.
Shell molds prepared by the method of the present invention also constitute part of the present invention, as do metal- and metal alloy components cast in these shell molds. Examples of such components are turbine engine parts.